The present invention relates to a lighting means compatible with a light intensifier night vision imaging system.
The present invention relates especially but not exclusively to lighting systems, lighting means and lighting or lit objects present in or on aircraft, for example instrument panel lighting systems, light scattering devices for ambient light in pilot""s cockpits, light indicators, luminous graphics display systems, position or navigation lights, landing lights, flight training lights, anti-collision lights, etc.
Recent years have seen the emergence in the market of aeronautical equipment for night vision systems to facilitate night flying and overcome the lack of the sensitivity of the human eye to light in the infrared range, namely at wavelengths of more than about 770 nm. These systems, generally known as NVIS (night vision imaging systems) are highly sensitive to infrared radiation up to wavelengths of about 900 nm. They usually take the form of goggles comprising two light intensifiers, each intensifier being comparable to a miniature video camera delivering an electronic image of the external environment. Even in the greatest darkness, a night imaging system delivers a clear, high-contrast monochromatic image of the external environment.
Very briefly, it may be recalled with reference to FIG. 1 that a light intensifier comprises a vacuum tube 2 having a photocathode 3 at its first end that converts the photons from the image received on its external face into an electron beam whose density and distribution are a function of the image. The electron beam is sent to a phosphor screen 4 positioned at the other end of the tube 2 by means of an amplifier plate 5. The amplifier plate 5 has numerous microchannels 6 covered with a secondary high-emission coating whose role is to greatly increase the number of electrons sent by the photocathode 3. The amplifier plate is driven by a circuit 7 called an xe2x80x9cautomatic gain controlxe2x80x9d circuit. This circuit 7 is a feedback circuit that optimizes the gain, namely the intensifying level, as a function of the ambient luminosity and gives a result comparable to the closing or opening of a diaphragm. Without this protection circuit, an increase in the ambient light energy in the band of sensitivity of the photocathode would prompt an immediate increase in the flow of electrons and would lower the sensitivity and resolution. The circuit 7 prompts the total extinction of the intensifier tube when there is a sharp variation in the radiant energy.
As light intensifiers have undergone various improvements since their appearance, there are now two types of light intensifiers using concurrent technologies on the market. These are GEN2 (second generation) and GEN3 (third generation) intensifiers. GEN3 tubes have a gallium arsenide photocathode and can be distinguished by their very high sensitivity to radiant energy, namely a sensitivity of about 1200 to 1800 xcexcA/lm depending on the models, and a fairly selective passband ranging from 600 nm (at the borderline limit between the yellow and the red wavelengths) to 900 nm. The GEN2 tubes have a tri-alkaline photocathode with lower sensitivity, of about 500 to 800 xcexcA/lm, and a wider passband ranging from 400 to 900 nm and covering the visible spectrum. For a clearer picture, the curves 10 and 11 of FIG. 2 respectively show the gain G of the tubes GEN2 a GEN3 as a function of the wavelength xcex. Despite their lower sensitivity, tri-alkaline photocathodes have a better signal-to-noise ratio than gallium arsenide photocathodes so that there are GEN2 type night vision systems that are equal to GEN3 type night vision systems in terms of resolution and image quality.
In practice, an essential goal to be achieved is that aircraft pilots should be able to use night vision goggles while continuing to be able to consult their panel instruments. This goal, which is essentially ergonomical, requires that two conditions should be met:
firstly, the intensifier tubes should not entirely mask the pilot""s visual field,
secondly, the lighting of the aircraft should not disturb the intensifier tubes by giving rise to parasitic halos or ghost images due to the reflection of illuminated objects on the windows of the cockpit.
With regard to the first condition, various ergonomical studies conducted in recent years have given rise to two types of night imaging goggles known as type I and type II goggles according to the MIL-L-85762A standard to which reference is made herein purely for reasons of convenience, the above-mentioned classification being frequently used by those skilled in the art. The I type goggles, designed for piloting helicopters, are fixed to the pilot""s helmet so that the two phosphor screens are before the pilot""s eyes at a minimum distance enabling him to see the panel instruments when he looks down. The type II goggles, designed for fixed-wing aircraft, work like a head-up display unit: the image delivered by the phosphor screens is projected before the pilot""s eyes by transparent lenses through which the panel instruments can be viewed in simultaneous juxtaposition.
Furthermore, the risks of interference between the light sources of the aircraft and the night vision goggles are eliminated by a retrofitting of the aircraft lighting system. This retrofitting operation essentially consists in securing all the light sources to a monochromatic color that is as far as possible from the red wavelengths band. Indeed, as can be seen in FIG. 2, the GEN2 or GEN3 type night imaging goggles do not have a passband limited to the infrared and have high sensitivity to the wavelengths in the red range, in a band that substantially covers 600 to 770 nm (herein with a view to simplicity, it is assumed that the red band also covers the orange and yellow wavelengths since, in practice, there is no purely monochromatic light: any orange or yellow light source inevitably includes a red component). In the prior art, the red wavelengths band is thus considered to be a critical band in which any emission of light is likely to greatly disturb night vision goggles by causing the activation of the automatic gain control circuit (namely the closing the electronic shutter). In particular, white incandescent lights are not allowed since they contain a high proportion of red and infrared light.
Thus, in practice, the retrofit of an aircraft illumination system consists in encapsulating the incandescent white lamps with lowpass attenuator filters and replacing the other white incandescent white lamps by light-emitting diodes or light-emitting panels scattering a narrow and green colored light also called an xe2x80x9caviation greenxe2x80x9d centered on the 555 nm. In general, the white incandescent lamps to be encapsulated are the yellow, orange and red lamps of warning indicators and alarm indicators. The incandescent white lamps need to be replaced by green light-emitting diodes and are for example green indicator lamps, used for the lighting of the instrument panel as well as backlighting lamps which, by transparency, reveal luminous graphic characters on an instrument panel. Finally, the lights used for ambient illumination are generally replaced by green light-emitting panels with which a mechanical scattering device with swiveling shutters is associated.
This kind of retrofitting of lighting means for an aircraft has various drawbacks. Firstly, it gives a greenish ambient light that very substantially attenuates the readability of the panel instruments and dilutes the colors. Thus, for example, the white or yellow, orange and red paint on the panel instrument packs (used for example to define and demarcate the operating modes of an engine) are respectively seen as green, light brown or dark brown. Again, the green lighting makes it difficult and tiresome to read the navigation maps. Furthermore, the red alarm indicators and those that have a red component like the yellow and orange indicators have mediocre luminosity and unsatisfactory coloring owing to the high absorption of the attenuator filter. Finally, yet another drawback of a retrofit operation of this kind is the high cost of the attenuator filters.
In the prior art, these drawbacks are considered to be inherent because it is sought to reconcile the infrared vision of the external environment with the natural vision of the panel instruments. It is furthermore considered that the xe2x80x9caviation greenxe2x80x9d color at 555 nm is the ideal color to reconcile the various requirements. Firstly, this green is distant enough from the red not to disturb the nightvision system. Secondly, the green light-emitting diodes (as well as the green light-emitting panels) are xe2x80x9ccleanxe2x80x9d and emit practically no energy in the red band, namely beyond 600 nm. Finally, green is the color where the sensitivity of the human eye is the maximum, so that it is preferred to the blue. More specifically, apart from its low sensitivity in the blue, the human eye has high remanence in the blue. This is considered to be incompatible with night vision.
Despite these various considerations which have led those skilled in the art to make the technological choices just described, the present invention is based on a surprising observation whose significance runs counter to prevailing assumptions and standards. According to this observation, a white color emitted by a light source working by electroluminescence in no way disturbs the presently known night vision systems, whether of the GEN2 type or of the GEN3 type. More specifically, the white light-emitting diodes as well as the white light-emitting panels available in the market of luminous components, normally intended for large-scale consumer applications, emit radiation that does not activate the automatic gain control circuit of a night vision system and does not even need to be filtered in the critical band of the red to offer these advantages. A non-disturbing radiation of this type however has a red component, but one with a low energy component. Here below, we shall see the explanations that may be given for this very advantageous compatibility of the white light-emitting sources with light intensifier systems. In any case, the discovery made by the Applicant has a considerable practical consequence for the field of aeronautics which is that it can offer aircraft pilots viewing comfort comparable to that of daylight while enabling the infrared display of the external environment by means of a light intensifier system without any risk of the untimely activation of an automatic gain control circuit.
Thus, the present invention proposes a lighting means compatible with a light intensifier night vision system comprising at least one light-emitting source of a white light with low radiant energy in the red wavelengths band.
The invention is made with a lighting means compatible with a light intensifier night imaging vision system whose particular feature is that it includes at least one light-emitting source of polychromatic white light with high radiant energy in the violet/blue wavelengths band, and with low residual energy in the red wavelengths band.
The polychromatic white light may furthermore have high radiant energy in the green/yellow and/or orange wavelengths bands with low residual energy in the red wavelengths band.
Preferably, the white light-emitting source has an emission spectrum comprising a dominant in the violet/blue wavelengths band and a dominant in the green/yellow wavelengths band.
Preferably, the white light-emitting source has a bichromatic-dominant emission spectrum with a violet/blue chrominance peak and a very wide range of chrominance from the green to the orange.
Preferably, the light-emitting source has an emission spectrum having a main peak wavelength of less than 492 nanometers, the main peak being a narrow high-intensity peak, and a secondary peak wavelength ranging from 492 to 622 nanometers, the secondary peak being a wide, medium-intensity peak, with very, low residual intensity at wavelengths of over 622 nanometers.
Advantageously, the white light-emitting source can give direct lighting, ambient lighting or indirect lighting or lighting without filtering in the red wavelengths band.
According to one embodiment, the white light-emitting source gives lighting guided in a translucent board of the instrument panel.
According to one embodiment, the light source is a white light-emitting diode.
For example, the light-emitting diode may be mounted on a screw-in or bayonet type socket. It may also be used instead of the standard incandescent lamp.
To light up the graphics of an instruments panel, the light-emitting diode is placed behind an opaque panel comprising transparent zones that form graphic characters.
To form a colored indicator, especially a green, yellow or red indicator, the light-emitting diode is covered with a colored hood that is not filtered in the red wavelengths band.
For ambient light, especially in a cockpit or an instruments panel, the lighting means according to the invention may include a ramp of white light-emitting diodes. The lighting means may also include a white light-emitting panel.
The present invention also relates to a means of lighting, especially for position indicators, landing lights, anti-collision lights or flight training lights in an aircraft comprising a plurality of light-emitting diodes arranged on a printed circuit.
Advantageously, the printed circuit is fixedly joined to a screw-in or bayonet type socket.
Advantageously, the polychromatic white light-emitting source is used with a light intensifier night vision imaging system with no filtering of the night imaging system in the visible wavelength bands.
The present invention also relates to an aircraft including a light-emitting means of the type described here above.
The present invention also relates to a lighting system comprising means of lighting in the visible range, means of lighting in the infrared range and switching means to make a choice between a lighting position in the visible range and a lighting position in the infrared range, characterized in that the means of lighting in the visible range include at least one light-emitting diode emitting a polychromatic white light with high radiant energy in the violet/blue wavelengths band and low residual energy in the red wavelengths band.
The present invention also relates to a method for retrofitting an aircraft lighting system comprising incandescent lamps to a light intensifier night vision imaging system in which at least a part of the incandescent lamps are replaced by light-emitting diodes emitting a polychromatic white light with high radiant energy in the violet/blue wavelengths band and low residual energy in the red wavelengths band.
Also, to retrofit a system of position lights or flight training lights comprising incandescent lamps, each incandescent lamp is replaced by a plurality of light-emitting diodes emitting a polychromatic white light with high radiant energy in the violet/blue band of wavelengths and low residual energy in the red wavelengths band.
The polychromatic white light of the system may furthermore have high radiant energy in the green/yellow wavelengths band and/or the orange wavelengths band with low residual energy in the red wavelengths band.
Advantageously, the light emitted by the white light-emitting diodes is not filtered in the red wavelengths band.